A 39-year-old otherwise healthy woman was referred to our center in June 2002 due to progressive deterioration of vision in her left eye and visual field restriction of about two months' duration. Ever since she gave birth in 2000, she had been amenorrheic but still lactating. Neurological examination revealed bitemporal hemianopsia. Her endocrine function was normal.

MRI scan revealed a heterogenous intrasellar mass measuring 28 x 21 x 19 mm with suprasellar expansion and marked compression of the optic chiasm, hyperintense on T1-weighed images (Figs. 1a, 1b) and isointense on T2-weighed images. The tumor penetrated into the left cavernous sinus narrowing the left internal carotid artery and extending into the middle cranial fossa. Gadolinium contrast enhancement of the dura lining the medial part of the middle cranial fossa and the clivus was noted.

The patient underwent transsphenoidal surgery. The sella contained abundant brownish masses of atypical consistency resembling a clotted hematoma. The pituitary gland of normal appearance was identified on the right side of the sella. A subtotal excision of this lesion was carried out with some of its mass left in the left cavernous sinus. The postoperative course was uneventful and the patient reported a significant improvement in vision. Owing to the presentation of the tissue found in the sella and substantial intraoperative bleeding, a four-vessel cerebral angiography was additionally performed revealing no vascular malformations in the circle of Willis.

MICROSCOPIC PATHOLOGY

The resected tissue samples were routinely processed and stained with hematoxylin and eosin (Fig. 1c). Histopathology revealed a tumor arising from pigment cells containing melanin with uneven pigment concentration. Presence of hemosiderin granules was also identified. The specimen consisted of fairly large polygonal cells with abundant light cytoplasm and conspicuous borders. The cells formed nests and bands, and the nuclei showed minor differences in size with few mitotic figures. Of note was the presence of large, conspicuous nucleoli, some of which were irregularly shaped. The cytoplasm contained fine granules of a brown pigment. The scanty connective tissue stroma featured numerous macrophages laden with brown-coloured material. Immunohistochemical staining was positive for HMB45, vimentin and S100 (Figs. 1d, 1e, 1f), while it was negative for CKAE1/AE3 and EMA. Staining for type IV collagen was positive around the tumor cell nests (Fig. 1g). MIB1 was <1% (Fig. 1h). All the pituitary hormones were negative. The electron microscopy specimens revealed premelanosomes and melanosomes in various, mainly early, stages of differentiation, fully mature single-membraned melanosomes, as well as abundant nucleoli and lack of desmosomes and microvilli (Figs. 1i, 1j). What is your diagnosis?